Electrochromic (EC) materials are a subset of the family of chromogenic materials, which includes photochromic materials, and thermochromic materials. These materials change their tinting level or opacity when exposed to light photochromic), heat (thermochromic), or an electric potential (electrochromic). Chromogenic materials have attracted widespread interest in applications relating to the transmission of light.
An early application for chromogenic materials was in sunglasses or prescription eyeglasses that darken when exposed to the sun. Such photochromic materials were first developed by researchers at Corning Incorporated in the late 1960s. Since that time, it has been recognized that chromogenic materials could potentially be used to produce window glass that can vary the amount of light transmitted, although the use of such materials is clearly not limited to that prospective application. Another likely application will be in the production of display devices. Indeed, EC technology is already employed in the displays of digital watches.
Several different distinct types of EC materials are known. Three primary types are: inorganic thin films, organic polymer films, and organic solutions. For many applications, the use of a liquid EC material is inconvenient, and as a result, inorganic thin films and organic polymer films appear to have more industrial applications.
For inorganic thin film-based EC devices, the EC layer is typically tungsten oxide (WO3). U.S. Pat. Nos. 5,598,293; 6,005,705; and 6,136,161 describe an inorganic thin film EC device based on a tungsten oxide EC layer. Other inorganic EC materials, such as molybdenum oxide, are also known. While many inorganic materials have been used as EC materials, difficulties in processing and a slow response time that is associated with many inorganic EC materials have created the need to develop different types of EC materials.
Conjugated, redox-active polymers represent one different type of EC material. These polymers (cathodic or anodic polymers) are inherently electrochromic and can be switched electrochemically (or chemically) between different color states. A family of redox-active copolymers are described in U.S. Pat. No. 5,883,220. Another family of nitrogen-based heterocyclic organic EC materials is described in U.S. Pat. No. 6,197,923. Research into still other types of organic film EC materials continues, in hopes of identifying or developing EC materials that will be useful in commercial applications such as EC windows. There still exists room for improvement and development of new types of EC organic polymer films, and methods of making EC organic polymer films.
The EC devices that have thus far been constructed employ relatively rigid substrates. Polymer films are readily deposited on glass, and it is typically employed for this purpose when fabricating EC devices. However, for certain applications, it would be preferable to produce an ECD that is entirely flexible. Since a flexible ECD that is actuated with an applied electrical potential must retain the spacing between the working and counter electrodes comprising the device, it is not trivial to construct such a device. A flexible ECD must be capable of retaining the seal that keeps an electrolyte between the two electrodes, even when the device is repeatedly flexed. The configuration must enable bending of the device without sustaining damage.